Mud pulsers are an integral component of directional drilling and measurement while drilling in downhole oil and gas operations. Mud pulsers use a variable orifice downhole to choke (positive pulser) or divert (negative pulser) the flow of drilling mud though the tool. The pulses are encoded in patterns that include data measured by a measurement-while-drilling (MWD) tool. For directional drilling, information such as azimuth, inclination, and tool face orientation are used to generate a pulse code. One or more pressure sensors at the surface detect the resulting pressure signal and decode it for use in steering the drill. Many other types of data may also be transmitted.
A typical MWD mud pulser generates pressure pulses at 0.5 to 3 Hz. Given the slow transmission rate, the time required to transmit any information can be significant—on the order of minutes per data point. Additionally, a mud pulse signal tends to dissipate as the length of the drill string increases. Pump noise can also interfere with the mud pulse signal, especially when the signal has attenuated during propagation through a long drill string. There is an extensive patent literature describing mud pulse tools and methods of interpreting the signals. For example, U.S. Pat. No. 6,421,298 to Beattie et al. (“the '298 Patent”) discloses a method for detecting these signals in the presence of pump noise.
A wide range of mud pulse telemetry systems have been developed. However, the most common systems in use are the basic positive or negative mud pulse systems that rely upon interpretation of the relative amplitude of the mud pressure signal. Tools that modulate the frequency of the mud pulse signal have also been developed. Some tools incorporate a rotary valve with variable rotation rate to modulate the pulse frequency.
There is a need to provide improved mud pulse telemetry that is capable of providing faster data rates and a signal that is more resistant to noise.